Often prospective clients will ask for the names of others you have worked with. Satisfied clients can be of enormous help to you by serving as a reference for prospective clients and customers. Start building a list of references as soon as you can and have their phone numbers handy to give out as needed.
Even if you know someone won’t care if you use them as a reference, always call to request permission. Then call when someone will be calling. No one should ever be surprised to receive a reference call.